1. Statement of the Technical Field
The inventive arrangements relate generally to microstrip antennas and more particularly to crossed slot fed microstrip antennas.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. Conventional circuit board substrates are generally formed by processes such as casting or spray coating which typically result in uniform substrate physical properties, including the dielectric constant.
For the purposes RF circuits, it is generally important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, signal reflections and inefficient power transfer can result. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting circuit performance relate to the dielectric constant (sometimes referred to as the relative permittivity or εr) and the loss tangent (sometimes referred to as the dissipation factor) of the dielectric substrate material. The relative permittivity, or dielectric constant, determines the propagation velocity of a signal in the substrate material, and therefore the electrical length of transmission lines and other components disposed on the substrate. The loss tangent determines the amount of loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits can be formed in many different ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes.
Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is approximately equal to √{square root over (L1/C1)}, where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the permittivity and permeability of the dielectric material(s) used to separate the transmission lines. Conventional substrate materials typically have a relative permeability of approximately 1.0.
In conventional RF designs, a substrate material is selected that has a single relative permittivity value and a single relative permeability value, the relative permeability value being about 1. Once the substrate material is selected, the line characteristic impedance value is generally exclusively set by controlling the geometry of the line.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate, such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors, such as copper, gold, or tantalum, which generally function as transmission lines (e.g. stripline or microstrip or twin-line) in the frequency ranges of interest. As noted, one problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is reasonably suitable for all of the various passive components, radiating elements and transmission line circuits to be formed on the board.
In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to be an electrical ¼ wave. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line or a radiator element can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a high relative permittivity, such as 50 to 100. However, the use of a dielectric with a high relative permittivity will generally result in a significant reduction in the radiation efficiency of the antenna.
Antenna elements are sometimes configured as microstrip antennas. Microstrip antennas are useful antennas since they generally require less space, are simpler, and are generally less expensive to manufacture as compared to other antenna types. In addition, importantly, microstrip antennas are highly compatible with printed-circuit technology.
One factor in constructing a high efficiency microstrip antenna is minimizing power loss, which may be caused by several factors including dielectric loss. Dielectric loss is generally due to the imperfect behavior of bound charges, and exists whenever a dielectric material is placed in a time varying electrical field. Dielectric loss generally increases with operating frequency. For example, the extent of dielectric loss for a microstrip patch antenna is primarily determined by the dielectric constant of the dielectric space between the radiator patch and the ground plane. Free space, or air for most purposes, has a relative dielectric constant approximately equal to one.
A dielectric material having a relative dielectric constant close to one is considered a “good” dielectric material. A good dielectric material exhibits low dielectric loss at the operating frequency of interest. Hence, when a dielectric material having a relative dielectric constant substantially equal to unity is used, the dielectric loss is effectively eliminated. Therefore, one method for maintaining high efficiency in a microstrip patch antenna system involves the use of a material having a low relative dielectric constant in the space between the radiator patch and the ground plane.
Furthermore, the use of a material with a lower relative dielectric constant permits the use of wider transmission lines that, in turn, reduce conductor losses and further improve the radiation efficiency of the microstrip antenna. However, the use of a dielectric material having a low dielectric constant can present certain disadvantages, such as the inability to efficiently focus radiated power from the feed line through the slot for slot fed antennas.
Microstrip antennas are sometimes designed to emit multi-polarizations, such as when a circularly polarized output is desired. Dual polarizations and quad polarizations are commonly used. In these cases, a crossed slot configuration may be formed. For example, two feed lines, each driving separate slots of the crossed slot can be phased 90 degrees apart to produce a circularly polarized output. Improved balance can be realized by driving the crossed slot with four feed lines, the feed lines phased 90 degrees apart from their nearest neighbors.
Unfortunately, the performance of crossed slot microstrip antennas is compromised through selection of a particular dielectric material which has a single uniform dielectric constant. A low dielectric constant is helpful to allow wider feed lines, and as a result lower resistive loss, and minimize dielectric induced line loss. However, a low dielectric constant dielectric material in the junction region between the slot and the feed generally results in poor antenna radiation efficiency due to poor coupling characteristics through the slot. As a result, a conventional dielectric material selected must necessarily compromise either the loss characteristics or the efficiency of the antenna.